Towards physiologically meaningful water-use efficiency estimates from eddy covariance data

Jürgen Knauer, Sönke Zaehle, Belinda E. Medlyn, Markus Reichstein, Christopher A. Williams, Mirco Migliavacca, Martin G. De Kauwe, Christiane Werner, Claudia Keitel, Pasi Kolari, Jean Marc Limousin, Maj Lena Linderson

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Intrinsic water-use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained from leaf-level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long-term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale-dependent and method-specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G1, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem-level estimates of G1: (i) non-transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non-closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within-canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G1 was sufficiently captured with a simple representation. G1 was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non-transpirational water fluxes. Uncertainties in the derived GPP and physiological within-canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC-derived water-use efficiency is interpreted in an ecophysiological context.

LanguageEnglish
Pages694-710
Number of pages17
JournalGlobal Change Biology
Volume24
Issue number2
DOIs
Publication statusPublished - Feb 2018

Fingerprint

eddy covariance
water use efficiency
Ecosystems
canopy
Water
ecosystem
gas exchange
energy balance
aerodynamics
Fluxes
Energy balance
water
productivity
net ecosystem exchange
flux measurement
latent heat flux
Aerodynamics
temperate forest
terrestrial ecosystem
Gases

Keywords

  • aerodynamic conductance
  • canopy gradients
  • eddy covariance
  • energy imbalance
  • intrinsic water-use efficiency
  • Penman–Monteith equation
  • slope parameter
  • surface conductance

Cite this

Knauer, J., Zaehle, S., Medlyn, B. E., Reichstein, M., Williams, C. A., Migliavacca, M., ... Linderson, M. L. (2018). Towards physiologically meaningful water-use efficiency estimates from eddy covariance data. Global Change Biology, 24(2), 694-710. https://doi.org/10.1111/gcb.13893
Knauer, Jürgen ; Zaehle, Sönke ; Medlyn, Belinda E. ; Reichstein, Markus ; Williams, Christopher A. ; Migliavacca, Mirco ; De Kauwe, Martin G. ; Werner, Christiane ; Keitel, Claudia ; Kolari, Pasi ; Limousin, Jean Marc ; Linderson, Maj Lena. / Towards physiologically meaningful water-use efficiency estimates from eddy covariance data. In: Global Change Biology. 2018 ; Vol. 24, No. 2. pp. 694-710.
@article{c943af3d6e9343a59117679c92291da5,
title = "Towards physiologically meaningful water-use efficiency estimates from eddy covariance data",
abstract = "Intrinsic water-use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained from leaf-level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long-term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale-dependent and method-specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G1, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem-level estimates of G1: (i) non-transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non-closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within-canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G1 was sufficiently captured with a simple representation. G1 was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non-transpirational water fluxes. Uncertainties in the derived GPP and physiological within-canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC-derived water-use efficiency is interpreted in an ecophysiological context.",
keywords = "aerodynamic conductance, canopy gradients, eddy covariance, energy imbalance, intrinsic water-use efficiency, Penman–Monteith equation, slope parameter, surface conductance",
author = "J{\"u}rgen Knauer and S{\"o}nke Zaehle and Medlyn, {Belinda E.} and Markus Reichstein and Williams, {Christopher A.} and Mirco Migliavacca and {De Kauwe}, {Martin G.} and Christiane Werner and Claudia Keitel and Pasi Kolari and Limousin, {Jean Marc} and Linderson, {Maj Lena}",
year = "2018",
month = "2",
doi = "10.1111/gcb.13893",
language = "English",
volume = "24",
pages = "694--710",
journal = "Global Change Biology",
issn = "1354-1013",
publisher = "Blackwell, Scientific",
number = "2",

}

Knauer, J, Zaehle, S, Medlyn, BE, Reichstein, M, Williams, CA, Migliavacca, M, De Kauwe, MG, Werner, C, Keitel, C, Kolari, P, Limousin, JM & Linderson, ML 2018, 'Towards physiologically meaningful water-use efficiency estimates from eddy covariance data', Global Change Biology, vol. 24, no. 2, pp. 694-710. https://doi.org/10.1111/gcb.13893

Towards physiologically meaningful water-use efficiency estimates from eddy covariance data. / Knauer, Jürgen; Zaehle, Sönke; Medlyn, Belinda E.; Reichstein, Markus; Williams, Christopher A.; Migliavacca, Mirco; De Kauwe, Martin G.; Werner, Christiane; Keitel, Claudia; Kolari, Pasi; Limousin, Jean Marc; Linderson, Maj Lena.

In: Global Change Biology, Vol. 24, No. 2, 02.2018, p. 694-710.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Towards physiologically meaningful water-use efficiency estimates from eddy covariance data

AU - Knauer, Jürgen

AU - Zaehle, Sönke

AU - Medlyn, Belinda E.

AU - Reichstein, Markus

AU - Williams, Christopher A.

AU - Migliavacca, Mirco

AU - De Kauwe, Martin G.

AU - Werner, Christiane

AU - Keitel, Claudia

AU - Kolari, Pasi

AU - Limousin, Jean Marc

AU - Linderson, Maj Lena

PY - 2018/2

Y1 - 2018/2

N2 - Intrinsic water-use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained from leaf-level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long-term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale-dependent and method-specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G1, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem-level estimates of G1: (i) non-transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non-closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within-canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G1 was sufficiently captured with a simple representation. G1 was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non-transpirational water fluxes. Uncertainties in the derived GPP and physiological within-canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC-derived water-use efficiency is interpreted in an ecophysiological context.

AB - Intrinsic water-use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained from leaf-level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long-term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale-dependent and method-specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G1, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem-level estimates of G1: (i) non-transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non-closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within-canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G1 was sufficiently captured with a simple representation. G1 was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non-transpirational water fluxes. Uncertainties in the derived GPP and physiological within-canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC-derived water-use efficiency is interpreted in an ecophysiological context.

KW - aerodynamic conductance

KW - canopy gradients

KW - eddy covariance

KW - energy imbalance

KW - intrinsic water-use efficiency

KW - Penman–Monteith equation

KW - slope parameter

KW - surface conductance

UR - http://www.scopus.com/inward/record.url?scp=85041375135&partnerID=8YFLogxK

U2 - 10.1111/gcb.13893

DO - 10.1111/gcb.13893

M3 - Article

VL - 24

SP - 694

EP - 710

JO - Global Change Biology

T2 - Global Change Biology

JF - Global Change Biology

SN - 1354-1013

IS - 2

ER -

Knauer J, Zaehle S, Medlyn BE, Reichstein M, Williams CA, Migliavacca M et al. Towards physiologically meaningful water-use efficiency estimates from eddy covariance data. Global Change Biology. 2018 Feb;24(2):694-710. https://doi.org/10.1111/gcb.13893